Measuring the ratio of CO2 efflux to O2 influx in tree stem respiration

In recent studies, the ratio of tree stem CO₂ efflux to O₂ influx has been defined as the apparent respiratory quotient (ARQ). The metabolism of carbohydrates, the putative respiratory substrate in trees, is expected to yield an ARQ of 1.0. However, previous studies have reported ARQ values ranging between 0.23 and 0.90. These interesting results may indicate internal transport of respired CO₂ within stems; yet no simple field applicable methods for ARQ measurement have been available. Here, we report on the assembly of a closed circulating system called 'Hampadah', which uses CO₂ and O₂ analyzers to measure air samples from stem chambers. We tested the performance of the Hampadah with samples from 36 trees (Tetragastris panamensis (Engl.) Kuntze). Additionally, we showed the feasibility of measuring ARQ directly from stem chambers, using portable CO₂ and O₂ sensors, in both discrete and continuous modes of operation. The Hampadah measurement proved to be consistent with CO₂ gas standards (R² = 0.999) and with O₂ determined by O₂/Ar measurements with a mass spectrometer (R² = 0.998). The Hampadah gave highly reproducible results for ARQ determination of field samples (±0.01 for duplicates). The portable sensors measurement showed good correlation with the Hampadah in measuring CO₂, O₂ and ARQ (n = 5, R² = 0.97, 0.98 and 0.91, respectively). We have demonstrated here that the Hampadah and the sensors' methods enable accurate ARQ measurements for both laboratory and field research.